1. Field of the Invention
The subject invention relates to systems for entertaining and informing passengers aboard aircraft and, more specifically, to wireless methods and apparatus for supplying audio information in several channels to passengers seated inside a metallic fuselage.
2. Disclosure Statement
In contemporary airline traffic, passengers are supplied with audio information for several reasons, including the communication of safety instructions, flight information and news and the provision of audio entertainment and sound accompaniment for motion pictures or video programs displayed during the flight. In practice, such audio information is distributed among the airline passengers in different channels for individual reception via headsets, so that passengers are enabled to effect selections among different music or other audio presentations, or to receive the audio accompaniment of a motion picture or video presentation they may be viewing, or to choose to be undisturbed by any of the audio information received by other passengers.
Two systems are currently in use for supplying audio information of the above mentioned type in several channels to seated commercial airline passengers. One of these employs wire harnesses extending from a central station in the aircraft to individual program selector and sound transducer units in armrests of passenger seats. The other system employs time division multiplexing to combine multiple audio channels for distribution over a coaxial cable system to passenger seat mounted decoders.
At the central station, electric signals oscillating in the audio frequency range and containing audio information in different channels are generated and applied to the wire harness system or multiplex encoder, in each respective system. At each armrest unit, a selector switch enables the passenger seated at that unit to select one of several active channels for listening. The electric signal of the selected channel may also be varied in amplitude through a passenger-actuated volume control.
The selected and volume-controlled electric signal is transduced to a corresponding sound signal for auditory reception by the selecting passenger. For this purpose, each participating passenger is typically provided with a headset. In principle, electric-to-sound transducers may be provided in the headsets supplied to the passengers. However, existing systems typically employ pneumatic heatsets, which are more economical to manufacture, easier to clean and sanitize between uses, and less vulnerable to theft than electric headsets, which would be more valuable and have more uses outside the aircraft. In the case of pneumatic headsets, a dual or stereo electric signal-to-sound signal transducer is located in the armrest unit and has a pair of plug-in openings for receiving a double barrel plug of the pneumatic headset. A pair of second conducting flexible tubes leads from that double barrel plug to a pair of different earpieces which are held against portions of the wearer's ears for high-fidelity listening.
In practice, these prior-art audio entertainment systems have been a source of severe trouble to the airlines, requiring a disproportionate amount of servicing and trouble-shooting. On the other hand, the type of audio information system herein under consideration is filling an increasing public need in terms of passenger information, edification, diversion and entertainment. Especially passengers beset by air fright are often calmed by their listening to a familiar or interesting program, while international travelers often find it useful to familiarize themselves with the language of a host country through their listening to video sound accompaniments, news or spoken programs.
By far the most troublesome component of conventional systems of the subject type has been the wire harness, displaying a particularly chronic vulnerability at the cabin wall/passenger seat interface or cabin floor/passenger seat interface.
Of course, a wireless approach has for a long time been employed in the communications industry whenever use of a wire system was impossible or inconvenient. However, anyone contemplating a wireless system for passenger entertainment inside an aircraft quickly would have been discouraged by a number of formidable obstacles. For one thing, it is difficult to cover the universally elongate space of the airplane passenger section uniformly with a wireless system. On the other hand, radio frequency signals or interference emitted by a high-flighing aircraft easily covers a huge space and large land and sea masses in a practically unobstructed manner, thereby interfering with a multitude of radio broadcasting, television, radio astronomy, radio navigation and security systems.
Also, if the most vulnerable part of prior-art systems, namely, the cabin wall or floor/passenger seat interface is to be avoided, the provision of an individual antenna for each seating unit becomes practically unavoidable in a wireless system. This in practice poses a very difficult problem, since airline passenger seats are subject to safety requirements, maintenance operations and cleaning procedures which in effect discourage the use of any antenna or other electronic equipment at any place other than the current location of the audio entertainment receptacle in the armrest of the passenger seat. However, from an overall point of view, that would appear to be the least suitable position for a receiving antenna, since the armrest includes metallic structural parts that would shield a built-in antenna against radio reception, while affording at best a very limited space for the placement of an antenna. Also, an armrest, along with adjacent portions of a seat, is naturally located in the region most likely shielded by the body of a seated airline passenger.
Of course, a traditional approach to problems of the latter type has been to increase power and, if possible, select a frequency so as to bring about penetration through unavoidable obstacles. By way of example, such an approach has been employed in the wireless paging field operating typically in buildings or over land surfaces. In an airborne situation, there are, however, definite low-level limits to such an approach, since any increase in transmitted power beyond a rather low level may spell potential interference with the aircraft's navigational and safety system.
Increased transmitter power and changes in transmitter frequency may also expose the navigational systems of other aircraft, as well as the operation of radio communication, television, radio astronomy and other radio frequency systems to interference either through the transmitted signals themselves or through one or more of their harmonics. Also, if several aircraft were to be equipped with wireless audio entertainment systems, it would be important to prevent mutual interference among such systems.
Another problem arises in connection with the transmitting antenna. From the point of view of conventional radio engineering, it would appear best to provide a dipole-type of antenna as the transmitting antenna for a wireless radio entertainment system along one of the bulkheads or class dividers running athwart the passenger cabin. However, this would not provide a uniform coverage of the passenger section at an acceptable power level.
In consequence, the prior art was unable to overcome the above mentioned disadvantages and obstacles, and to meet the above mentioned needs.
In this respect, even measures adopted or proposals made in other fields do not offer much concrete assistance to the person having ordinary skill in the subject art. For instance, antennas in the form of wires extending along underground or underwater tunnels, such as automobile traffic or railroad tunnels, have been used for years to maintain radio broadcast reception or radio communication with respect to automobiles, trains or other vehicles.
For instance, the article by R. A. Farmer and N. H. Shepherd, "Guided Radiation. The Key to Tunnel Talking", IEEE Transactions on VEHICULAR COMMUNICATIONS, Vol. VC-14, No. 1 (March 1965) pp. 93 to 102, discusses "indoor space" two-way mobile radio communication at 160 MHz. This, however, is within less than 10% of the frequency of 150 MHz which has been designated as "almost the worst frequency" which could be chosen for tunnel transmission in an article by N. Monk and H. S. Winbigler, entitled "Communication with Moving Trains in Tunnels", IRE Transactions on VEHICULAR COMMUNICATIONS, Vol. PGVC-7 (December 1956) pp. 21 to 28, at 24/25. Also, the wavelength of 160 MHz would approach values at which substantial amounts of the transmitted energy could penetrate the airplane windows, thereby raising the danger of interference with systems, such as certain maritime and railroad communication systems, operating at that frequency. The latter IRE article also makes the point that there is a change-over from free-space to waveguide transmission at a critical cut-off frequency, considering the tunnel as a circular waveguide. On page 24, that article designates such cut-off frequencies as being in the order of 50 MHz. Even though FIG. 7 of that article shows the effect of cut-off and transmission change-over in terms of a tunnel occupied by a train, that FIG. 7 appears to demonstrate strongly that frequencies occurring in a cut-off or change-over region would not be suitable for transmission purposes.
In terms of a practically "empty tunnel" such as constituted by the metallic fuselage of an aircraft, it would thus appear from the IRE article that no suitable audio system transmission frequency above television channel 4 and below the aeronautical marker beacon and radio astronomy band could be found.
Conventional know-how on "indoor space" or tunnel transmission thus would appear to have a discouraging effect on a person of average skill, as far as any transfer of such transmission technology to the transmission of information in the passenger section of aircraft is concerned.
Another problem arises from the fact that modern airline entertainment systems require as many as a dozen program channels, which would raise considerable problems if wireless transmission of the channels through the aircraft were attempted at high-fidelity quality.
There thus exists a need to reduce transmission bandwidth requirements in systems of the type here under consideration, as well as in wireless multichannel systems in general. Especially aboard aircraft, this is paralleled by a need to minimize bulk and weight of multiplexing systems.